low pressure rich sour gas treating

[Jibran]

I had tried it in the Chemical Process Plant Design & Operations Area, however, didn't have luck yet, thought should post in the Chemical Process Engineering area as well.

I have a situation where we are compressing low pressure associated/flashed/vent wet sour gas, extracting LPG, C4 & C5+ liquids and using the residue gas stream for the fuel use. The gas rich in heavy hyrocarbons need to be sweetened before NGL recovery. Amine type treatment seems suitable. Gas parameters are:

Inlet CO2: less than 1 mole%
H2S: below 500 ppmv
Pressure, psig: 90 - 100
Temperature oF: 100 - 110
Mol. Wt. above 40
Gas Flow, MMSCFD: 50

As such CO2 removal is not necessary. Recovered acid gas from the amine unit would be incinerated. I was wondering if some of you have a similar experience (low pressure, low H2S/CO2 ratio, NGL rich). Any operating problems? What generic amine type would be the most suitable? Client based on their past experience is not very much in liking of MEA. Any suggestions? Thanks a lot for your time!

 

[Montemayor]

Jibran:

I just got back home from Arizona.

I believe you can use DEA, MDEA, or DGA - among some of the more "classic" absorbing solutions for sweetening natural gas. There are other available absorbing solutions and processes - it would take more than this forum to discuss them, however. Your client not liking MEA is one thing, but what does he/she propose? The license, corrosion characteristics, capital requirements, energy requirements (especially reboiler duty), chemical costs, are just some of the variables that have to be taken into consideration. My recommendation (if I were to give one with so little input) for the proper process couldn't be taken seriously based on just your question. I would suggest that you consult with experts who design and fabricate these type of units for a living, such as:

http://www.gastecheng.com/sweetening_units.html

by inquiring with firms such as these, you'll get more in-depth information about just what are your options for the proposed cleanup.

Based on what you've given as basic data, I would select an amine process (probably MDEA) - but then, you'd come back and state that you don't like it. It's not enough to select the type of process; you must (if you want to make money) carefully do an economic evaluation on the various options in order to optimize the fit of process to your application. For example, the amines are going to be inherently corrosive and demanding in reboiler steam duty. If you don't have cheap, available LP steam (35-50 psig), then kiss off on the amines and look elsewhere. You will find that other processes also have their characteristic trade-offs. There are no exceptions to the trade-off requirement - you simply have to find the best, acceptable fit.

Art Montemayor
Spring, TX

 

[laver]

I've worked in a refinery with DEA and currently work in a gas plant using MDEA. I support Montemayor's suggestions in that the best thing to do is contact the pro's on this one. I would suggest an amine supplier of your choice, they would be more then happy to explain which amine is the best for your process. I would say the information you have already gathered would be more then enough for the supplyer to provide you with an amine of choice. Because CO2 removal is not of great importance I would stick with DEA, but again the best thing to do is consult with an amine pro.

 

[MortenA]

Since your gas flow and H2S concentration is not very high LO-CAT (iron reduction) may be an economical advantages?

The vendor (check this site):

http://www.gtp-merichem.com

claims that up to 30 tons/day can be favorable.

Your flow is 50,000,000 SCFD * 0.837 mol/SCFD* 18 g/mol*500 ppm == 0.4 t/d. (all metric sorry)

The technology is best suited for low pressure and intermediate H2S concentration.

Scavenger chemicals might also be usefull if you fuel gas user can handle this?

Best regards

Morten

 

[PHP78]

Hi
I am PhD student working on such kind of problem. Well i have to firtst clarify that i dont have much industry experience and my work is synthesis of such flowsheets.

well.. one of the main reason for your client to avoid MEA is it is highly corrosive and also thermodynamically, it is not very attractive because of high heat of reaction etc (should not be a problem in your bcz of low conc.).
If you are going for amines then MDEA is very good choice for your requirements. MDEA is used for selective removal of H2S as its reaction with CO2 is very slow. for H2S, it is just a proton transfer and instantaneous. Also MDEA has less corrosion problems.
As pointed out by other people in this forum, first you should clarify from your client why are their limitations/considerations etc... , then decide which process to use (i think that you may end up in amines as ppl know the process behaviour)... use some simulation techniques (tsweet, hysys etc) and see the effects of different parameters..


Prashant

 

[Jibran]

Thanks a lot Montemayor, MortenA, PHP78. I am definitely in agreement with you all to contact the pros. In fact, I did and their opinion is inclined towards MDEA based solvents. I have prior operating experience with BASF aMDEA for an Ammonia plant, however, the objective in Ammonia plant is different (CO2 removal from synthesis gas at high pressure). PHP78 is right about selectivity of MDEA towards H2S. I probably didn't put my question properly. Let me put it in the perspective:

1): There is slight uncertainity in the Feed Gas composition as no real gas sampling has been accomplished yet...which should be the foremost step!
2): Feed gas is rich in NGLs (could be upto 0.5 BBL/1000 SCF), MDEA absorption towards NGL is typically higher, foaming problems....??
3): Feed gas could contain mercaptans and other organic sulfur species in trace amounts <1 - 10 ppmv (which could be removed in downstream liquid (NGL) treating). However, capture of these species, earlier the better...!!
4): Energy is cheap (plenty of fuel gas available which otherwise would be flared anyway...!!)
5): No infrastructure available to handle any byproducts (such as solid sulfur) that is why acid gas recovered could be incinerated (provided it complies with SO2 environmental guidelines).
6): Client's disliking of MEA is probably because of previously known problems of corrosion, amine losses, foaming, amine degradation (& reclammation requirements adding more complexity to the operation and handling wastes..!!).

I was interested in knowing
1): Whether MDEA can achieve sweet gas spec down to pipeline spec of <4ppmv at such low partial pressures in real life (simulation says, it does..!!)?
2): Does anyone have operating /design experience with a gas stream of similar parameters, what are their comments?
3): What do they use (MEA, DGA, or MDEA(or MDEA based))?
4): Any other comments or operational problems??

To me, MEA at very dilute concentrations (<=15%) should do the job, however, I neeed to convince the client with strong arguments or more evidence...!! They are the client...!!

Luv you all.

 

[Montemayor]

Jibran:

Now that we have some more basic data, I can suggest the following:

1) I would use an MEA 12% solution and use carbon steel construction except on the reboiler tubes and MEA exchanger(s);
2) The existance of "cheap" energy isn't of direct help; I need 35 psig saturated steam; you have only fuel available. I don't install direct-fired heater/reboilers on Amine service. The heat flux is too high and the solution decomposition and corrosion is very elevated.
3) I would install a small, fire tube steam generator and generate the 35 psig steam, dedicated solely to the reboiler (& redistillation still) and return the condensate by gravity back to the boiler (the kettle reboiler is elevated for that). This gives no moving parts on the steam/condensate system.
4) MEA can be redistilled very easily and the corrosiveness is reduced/eliminated; plus the chemical consumption is drastically reduced.
5) 12% MEA does not corrode Carbon Steel; I have designed and operated this type of arrangement for years, without any corrosion at all.

The more dilute the MEA solution, the higher the circulation rates required and the higher the reboiler duty, & the equipment sizing. However, this avoids any corrosion and practically eliminates potential foaming problems in the absorber/stripper. The process behaves very benign and gives no process upsets, corrosion, or problems. The units I've had like this ran for over 20 years with the original equipment. However, the trade-off is energy consumption being higher than normal and carbon steel equipment being larger. However, I found the savings in chemical consumption alone justified the use of lower Amine concentration solution.

I use Stainless Steel tubes on hot amine service as a general rule and it has paid off through the years with no leaks, replacements, or problems. If you want a long-term, trouble-free unit, that is what I would do.

MEA can be redistilled. DEA and MDEA cannot, I believe. So this gives you an edge with the MEA. Plus MEA is cheaper, I believe.

I hope this experience helps.

Art Montemayor
Spring, TX

 

[SeanB]

I have participated in the design on an "Offgas Scrubber" that is very similar to your application. We are using MEA in the scrubber. As you mentioned there can be foaming problems but our guidelines indicate that if you keep the lean MEA feeding the tower at least 10 deg F above the hydrocarbon temperature you can prevent the foaming.

 

[dcasto]

We operate several low pressure rich gas amine systems. At pressures under 200 psig, you have to use a primary amine, MEA or DGA. The heavy hydrocabons dissolve in DGA and would be a real problem for your gas. I'd stick with MEA and go with a low concentration, under 15% solution and most of all keep the loading under .35 mole acid gas to mole amine. this is a 200 gpm solution circulation and under 10 MMBTU/hr heat. Indirect oil heater or steam would be best. Two 100 gpm units with direct fired heaters would also work.

Like Art states, carbon steel in most of the process is acceptable. A good flash tank and reclaimer will be required too.

 

[Jibran]

I am very glad that you guys spent time and came up with suggestions and comments. I think, we all believe, MEA to be a more suitable choice for this service than other solvents. No one has came up yet with MDEA experience with similar servive.

I am glad to be part of such a nice community. Thanks to all of you (Montemayor,SeanB & dcasto).
Jibran

 

[PHP78]

Jibran
it is nice community here and people are ready to help.
still i will suggest you that you should compare both options MEA and MDEA. why I am saying it is because.... though MDEA is known for long while, real applications came in large number in last 10 years. Thats what i can see from the research papers and some conferences (for example see some websites like gas processors association GPA, TSWEET software website etc).
People in this thread has suggested MEA based on their experiences but nobody has compared it with other amines like MDEA. In my opinion you should compare both amines initially on simulator and compare on cost, corrosion etc issues. and if it turns out to be MEA, then go for it as you have some discussion here based on previous experience. if MDEA is choice then look for other data.

Prashant

 

[bspooner]

Unfortunately MDEA will be of little use in this application because at low pressure it does not remove H2S very well. You will probably have the best luck with DGA. It will degrade in the presence of CO2 but can be reclaimed.

Ben Spooner, P. Eng
Process Engineer
Amine Experts Inc
Calgary, Alberta

 

[TexasPE]

Contact Huntsman Chemicals and register for their web site at

http://www.huntsman.com/performance_chemicals/index.cfm?PageID=887.

Once registered you can download andf use their "process simulator" called QuickTreat® Estimator which can do used to evaluate their amines for scoping type studies.